1. Field of the Invention
The present invention relates to high-strength joined parts of Ni-Ti alloys, in particular, a Ni-Ti alloy component having shape memory characteristics or superelasticity characteristics joined to a different metal alloy, and a joining method therefor.
2. Description of the Related Art
The shape memory effect and the superelasticity effect aforementioned both result from a thermoelastic Martensitic transformation with small transformation hysteresis. The shape memory effect is a phenomenon wherein a deformation which takes place in the Martensite temperature range returns to the original undeformed shape when heated above the Martensite inverse transformation temperature (Austenite transformation). On the other hand, the superelasticity effect is a phenomenon wherein a material deformed in the Austenite temperature range recovers elastically as much as 8% deformation strain without the heating as above by the stress-induced Martensite transformation accompanied with deformation.
Among the technologically advanced materials exhibiting such shape memory and superelasticity effects are certain Ni-Ti alloys. Products exhibiting shape memory effect prepared from these Ni-Ti alloys include blow-out ports of air controllers, pressure-regulating valves of rice-cooking jars, medical bone plates, etc. Products wherein the superelasticity effect is utilized in practice include orthodontic wires, brassiere cup wires, spectacle frames, medical guide wires, etc.
Further development in related technologies often requires creating composite structures. Preparation of such structures requires a joining technique for joining Ni-Ti alloys with different metals.
The Ni-Ti alloys of concern are rather specific alloys exhibiting ductility while yet being intermetallic compounds. The compositional range is in the vicinity of a ratio of Ni to Ti of 1:1 in terms of atomic ratio, occasionally with an extremely narrow range of additional metals added thereto. If this ratio shifts slightly, the alloys become brittle rapidly. Moreover, since these alloys have high activity in the chemical sense, they react easily with many metals such as Fe, Cu and Ni to produce a brittle reaction phase. The production of this reaction phase makes welding and soldering Ni-Ti alloys to different metal materials difficult.
Conventionally, with respect to the joining of Ni-Ti alloys, Ni-Ti alloys can be easily joined to themselves by fusion welding methods such as laser welding, TIG welding and electronic beam welding, or by pressure welding methods such as upset butt welding, flash butt welding and friction pressure welding. Upset butt welding is generally considered superior with respect to strength.
However, techniques allowing Ni-Ti alloys and different metals other than said alloys to be directly welded while maintaining high strength has not previously been known. This is due to the fact that because of the high activity of Ni-Ti alloys, these alloys react easily with mating metals such as Fe, Cu, Ni, etc. during welding to produce a brittle reaction phase. Regardless of which welding methods aforementioned might be used, a brittle reaction phase is produced in the joined area, making it impossible to obtain the strength required for practical use.
For this reason, the direct welding of Ni-Ti alloys with different metals has been said to be impossible. As a substitute, the following two methods have been adopted as conventional joining techniques.
One such method is mechanical joining, for example, by such means as crimping with rivets, etc. and fastening with screws, methods which have the defect that the joined parts became large and subject to becoming loose due to repeated working.
Another method is plating/soldering. In this method, the joining face of the Ni-Ti alloy component is plated beforehand with an easily solderable metal, e.g. nickel, copper alloy or the like, and soldering with the mating component is performed on this plating, thereby preventing the reaction between the Ni-Ti alloy and the fusible metal brazing material, thereby achieving a stabilized joint. Because of the dependence of the joint strength on the adhesive strength of plating to the Ni-Ti substrate, however, joining of parts expected to be subject to high stress was impossible.